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Old 27th May 2008, 02:53 PM   #491
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Default EL84 fixed bias

Ian,

On the CCS, there is a 100uf 63v to ground. You marked the (+) to the LED side and (-) to ground. Please clarify.
Thanks

Albert
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Old 27th May 2008, 11:48 PM   #492
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Albert,
Well spotted - the capacitor IS drawn backward. -ve terminal goes to the LED side and +ve terminal goes to 0V.
Cheers,
Ian
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Old 28th May 2008, 11:15 AM   #493
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this looks great, i think i might attempt this build someday.
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Old 29th May 2008, 09:35 PM   #494
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Ian: Are you planning on posting a schematic for the monoblock huey power supply?

I have read your description of the mono power supply several times, but being a total newbie a schematic picture is worth 1000 words.....
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Old 5th June 2008, 06:40 PM   #495
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Default Fixed bias baby huey

Ian,

I have my fixed bias baby huey up for testing with the following numbers:

B+ 311v
plate 1 & 2 303v
G2 1 & 2 306v
Bias both 0.40v

+51v for ZVN0545A
B- 40v

On 12ax7 plate 1 61.5v
plate 2 -3.2v ( this is something not right )

Both green and blue LEDs don't come on. I did pull those out and check. They are ok.

Thanks
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Old 6th June 2008, 12:14 AM   #496
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Albertli,

The only way you could have a negative voltage on the anode of that 12AX7 is a wiring error or shorted coupling cap across to the ZVN0545.

Also the 2 LEDS MUST be ON for the CCS to work. The 0V to resistor, through 2 LEDs to -ve rail is an easy path to check. If you have CCS transistors of wrong type (PNP instead of NPN) or wired wrongly they could have a forward biased junction shorting the current around the LEDs in which case you would measure 0.7 Volts across the LEDs instead of about 1.8 to 2.0 V for the green and 2.3V for the blue.

I would be doing a full check of the front end wiring, looking for something not right.

Cheers,
Ian
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Old 6th June 2008, 06:21 AM   #497
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Ian,

Thanks for the quick reply. I certainly will check the front end wiring tomorrow.
Being a complete newbie, I did follow closely by using the exact parts ( NPN BC546B ) as of your schematic. Now the problem is I don't know how to check if they are ok.
The coupling caps are rated at 100v, so I thought it should be fine for this application.

Regards

Albert
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Old 6th June 2008, 06:37 AM   #498
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Albert,

100V rated coupling capacitors are NO GOOD.
At power on, before the tubes start conducting you may have as much as 350 or even 370 Volts DC on the anodes of the 12AX7. The other side of the caps have negative bias voltage on them. There is probably a total of around 400V across the caps. Even when the amp is operating correctly (when the 12AX7 anode volts will drop down) you will have about 200V across the coupling caps. They are almost certainly Cactus or whatever the expression to describe "gone to god" is in your part of the world. You will need to replace them. I would use 630V rated caps. Do this before chasing anything else.

Note: A "Cactus" Cap would cetainly explain that -3.2 volts you saw on one 12AX7 anode.

Cheers,
Ian
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Old 6th June 2008, 05:39 PM   #499
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Ian,

I replaced the coupling caps to .015 630v and have the measurement below:

1). both cathode of 12ax7 around -4v

2). across green led -18.2v

3). across blue led -7.8v

I have -7.5v between the coupling cap and 220R. -10.8 after the 12v zener.

Albert

BTW the supplies remain the same +51v and -40v.
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Old 10th June 2008, 03:41 AM   #500
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Albert,
It looks like you've fixed the coupling cap problems, those voltages you quote for "between the coupling cap and the 220R" and the "after the zener" (that is on the source of the ZVN0545A) look good.

You still have a Current Source problem however.

Working from your -40V supply:
1) You should have approx. 1mA through the CCS transistors PLUS about 2.5 to 3 mA through the LED biasing chain. That is approx 3.5 to 4mA total.
That means an approx. 8 volt drop across the 2K2 to give about -32V across the 100uF capacitor.

2) There should be 1.8 to 2.0 volt drop across the 1K8
3) There should be 2.5V drop across the blue led and 2.0V across the green led leaving 26 or 27 volts across the 10K resistor to ground (this resistor sets the led bias chain current).

Junction of the Emitter of the "bottom" transistor and the 1K8 should be at about -30V

Emitter of the "top" transistor should be at about -27V.

Pull the 12AX7 from its socket and work on the CCS till it looks right.
The LEDS should both be ON.
Check for transistors and LEDs incorrectly wired or blown up.

Fault finding tricks:
1) Short across the green LED and make sure the BLUE LED lights. If not, its in backward or the bottom transistor is incorrectly wired or blown.
Once that is going:
Remove the short across the green LED - check that Green LED is ON, if not its in backward or the top transistor is incorrectly wired or blown.

Good luck,
Ian
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